Subject: Re: [Patch] Fix sys_time to handle intra-tick correction
From: john stultz <johnstul@us.ibm.com>
To: Vinay Sridhar <vinay@linux.vnet.ibm.com>
Cc: linux-kernel@vger.kernel.org
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Date: Tue, 09 Dec 2008 18:25:28 -0800
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On Tue, 2008-12-09 at 16:24 -0800, john stultz wrote:
> On Tue, 2008-12-09 at 17:28 +0530, Vinay Sridhar wrote:
> > Hi All,
> > 
> > This fix changes sys_time to use do_gettimeofday instead of get_seconds.
> > Running the stime01 test from LTP triggers this error. Calling sys_stime
> > and then calling sys_time causes this. "do_settimeofday" subtracts the
> > nsec offset from the nsec value(0 in this case) passed to it.
> > Subsequently, "set_normalized_timespec" modifies sec and nsec
> > accordingly. This compensation is handled in the do_gettimeofday path.
> > However, sys_time does not handle this case and reports an incorrect
> > seconds value.
> > 
> > 
> > signed-off by : Vinay Sridhar <vinay@linux.vnet.ibm.com>
> 
> This does fix the symmetry by using the same interfaces on both sides,
> so it seems reasonable to me. However, since do_gettimeofday reads the
> clocksource hardware, this may have a slight performance impact to
> time() users.
> 
> I know privately I said this patch looked fine earlier, so sorry for the
> mixed messages, but I really think we should be able to use
> get_seconds() without this problem.
> 
> Looking more carefully at the code in do_settimeofday(), we should
> accumulate the left over cycles, then we set the current time to the
> specified time and update the xtime_cache.
> 
> So I'm not sure if we're really fixing the right issue here. Further,
> running on the x86_64 system, I don't see this issue crop up at all (for
> either 64 or 32 bit binaries). I think you mentioned privately that this
> was seen on i386, so maybe we need to dig just a bit deeper before going
> with this fix.
> 
> And sorry again for running hot then cold on this patch. I just didn't
> look deeply enough at the issue the first time around. :(

Hey Vinay,
	So I spent some time checking various kernels and it seems this issue
is already resolved upstream by the clocksource_forward_now() change:
9a055117d3d9cb562f83f8d4cd88772761f4cab0

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=9a055117d3d9cb562f83f8d4cd88772761f4cab0

thanks
-john


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